Friday, 10 June 2016

Black hole dark matter

The idea that dark matter is made of primordial black holes is very old but has always been in the backwater of particle physics. The WIMP or asymmetric dark matter paradigms are preferred for several reasons such as calculability, observational opportunities, and a more direct connection to cherished theories beyond the Standard Model. But in the recent months there has been more interest, triggered in part by the LIGO observations of black hole binary mergers. In the first observed event, the mass of each of the black holes was estimated at around 30 solar masses. While such a system may well be of boring astrophysical origin, it is somewhat unexpected because typical black holes we come across in everyday life are either a bit smaller (around one solar mass) or much larger (supermassive black hole in the galactic center). On the other hand, if the dark matter halo were made of black holes, scattering processes would sometimes create short-lived binary systems. Assuming a significant fraction of dark matter in the universe is made of primordial black holes, this paper estimated that the rate of merger processes is in the right ballpark to explain the LIGO events.

Primordial black holes can form from large density fluctuations in the early universe. On the largest observable scales the universe is incredibly homogenous, as witnessed by the uniform temperature of the Cosmic Microwave Background over the entire sky. However on smaller scales the primordial inhomogeneities could be much larger without contradicting observations. From the fundamental point of view, large density fluctuations may be generated by several distinct mechanism, for example during the final stages of inflation in the waterfall phase in the hybrid inflation scenario. While it is rather generic that this or similar process may seed black hole formation in the radiation-dominated era, severe fine-tuning is required to produce the right amount of black holes and ensure that the resulting universe resembles the one we know.

All in all, it's fair to say that the scenario where all or a significant fraction of dark matter is made of primordial black holes is not completely absurd. Moreover, one typically expects the masses to span a fairly narrow range. Could it be that the LIGO events is the first indirect detection of dark matter made of O(10)-solar-mass black holes? One problem with this scenario is that it is excluded, as can be seen in the plot. Black holes sloshing through the early dense universe accrete the surrounding matter and produce X-rays which could ionize atoms and disrupt the Cosmic Microwave Background. In the 10-100 solar mass range relevant for LIGO this effect currently gives the strongest constraint on primordial black holes: according to this paper they are allowed to constitute not more than 0.01% of the total dark matter abundance. In astrophysics, however, not only signals but also constraints should be taken with a grain of salt. In this particular case, the word in town is that the derivation contains a numerical error and that the corrected limit is 2 orders of magnitude less severe than what's shown in the plot. Moreover, this limit strongly depends on the model of accretion, and more favorable assumptions may buy another order of magnitude or two. All in all, the possibility of dark matter made of primordial black hole in the 10-100 solar mass range should not be completely discarded yet. Another possibility is that black holes make only a small fraction of dark matter, but the merger rate is faster, closer to the estimate of this paper.

Assuming this is the true scenario, how will we know? Direct detection of black holes is discouraged, while the usual cosmic ray signals are absent. Instead, in most of the mass range, the best probes of primordial black holes are various lensing observations. For LIGO black holes, progress may be made via observations of fast radio bursts. These are strong radio signals of (probably) extragalactic origin and millisecond duration. The radio signal passing near a O(10)-solar-mass black hole could be strongly lensed, leading to repeated signals detected on Earth with an observable time delay. In the near future we should observe hundreds of such repeated bursts, or obtain new strong constraints on primordial black holes in the interesting mass ballpark. Gravitational wave astronomy may offer another way. When more statistics is accumulated, we will be able to say something about the spatial distributions of the merger events. Primordial black holes should be distributed like dark matter halos, whereas astrophysical black holes should be correlated with luminous galaxies. Also, the typical eccentricity of the astrophysical black hole binaries should be different. With some luck, the primordial black hole dark matter scenario may be vindicated or robustly excluded in the near future.

34 comments:

AFAIR astrophysical black holes could not be lighter that ≈3 M_Sun otherwise it would stay as neutron star. And 30 M_sun black hole doesn't looks too strange to me while 300 would be very very strange. There're no stars this heavy

Also I don't think we'll have enough of angular resolution to distinguish between mergers in halo disk. At 400Mpc giant galaxy is about 30 arcsec and we'll be happy to have few degrees of angular resolution I think.

One solar mass black hole would indeed be something very strange. I don't see any astrophysical creation mechanism for them. A 300 M_Sun black hole wouldn't necessarily be that strange though, you could expect to find them (and there are some claims of discovery, though AFAIK nothing confirmed) in very dense stellar clusters.

My astrophysics colleagues seem to be of the opinion that the exclusions on a whole bunch of primordial black holes of the mass that LIGO observed may not be 100% airtight, but they're good and will soon get better.

The empirical evidence for MACHO objects discovered via microlensing has held up for quite a while. The mass range of these objects is on the order of 0.5 solar mass and there appear to be more of these mystery objects than there are stars in our Galaxy.

Astrophysicists have noted that PBHs may be the best candidate for these MACHOs.

While it is true that their estimated abundance currently has an upper limit of 20% of the Dark Matter mass, this DM candidate should not be summarily dismissed, especially since ongoing research promises to reduce the uncertainty in their abundance estimates and their physical characteristics.

I'm actually a little surprised that the count of gravitational wave detections still stands at one. Remember those rumors about other candidate events supposedly found alongside The One? Is it going to turn out to be just one every year or two? Because that's less fun than what I was hoping for.

Is no one else concerned about the possibility that the LIGO "observation" might be something that has accidentally fallen out of their stack of fake signals, and no one has noticed yet?

Computer software can contain bugs, which can sometimes be hard to detect, so the incorporation of a system with the capability of introducing fake signals into the data is a very serious flaw in the design of the experiment.

The very fact that during the "fake signal" runs,

http://www.ligo.org/news/blind-injection.php

the vast majority of the members of the LIGO collaboration have no idea whether the signal is genuine, or a deliberately introduced fake signal, shows that the organization of the LIGO collaboration is too opaque.

Transparency and scrutiny are fundamental requirements, and should never be sacrificed for "blindness".

RE: E. Goldfain's comments, I don't know of papers connecting PBHs to CMB properties, but consider the following regarding the cosmic IR background and the cosmic X-ray background.

Kashlinsky has a new paper published in ApJ (24 May 2016) that discusses a possible relationship among these topics.

http://arxiv.org/abs/1605.04023

Dark Matter in the form of primordial black holes may have generated the gravitational waves recently detected by LIGO. If such PBHs constitute a significant fraction of the Dark Matter, then this might explain the unexpected irregularity of the Cosmic Infrared Background, and also the unexpected finding that the CIB and the Cosmic X-ray Background have approximately correlated irregularities.

Kashlinsky's idea can be tested via its prediction that LIGO should discover additional gravitational wave events corresponding to mergers of BHs with masses of roughly 30 solar mass.

Supermassive black holes have to come from somewhere - there have to be smaller black holes around, everything down to the mass scale where black holes form.

Xezlec, the count is still at one highly significant event and one not that significant event because LIGO didn't publish the analysis of larger datasets yet. We have two events in 16 days, and the sensitivity is expected to improve by another factor 3 in the near future. The Einstein Telescope should then see too many events to look at all of them manually.

@Chris: The system that can inject fake signals was not even operational at the time of detection. But even if it would have been: the people who operate it know extremely well when they use it.

Equally intriguing would be to consider that some of the promising non-baryonic models of Dark Matter (such as the superfluid Dark Matter model) and the PBH hypothesis share a common origin. What we currently view as incompatible theories may have a deeper physical foundation, rooted perhaps in the un-conventional geometry of spacetime in primordial times.

Just out of curiosity: imagine such a mini black hole coming to rest in the middle of the sun or, more extremely, a neutron star. At what black hole mass would the mass loss due to Hawking radiation exactly outweigh mass gain due to infalling matter?

Dynamically a bag of fine sand would behave quite differently from a sack of large rocks (the granule mass difference in this case would be in the 10^dozens). Should not we be able to detect such differences from observing movement of visible objects?

mfb, well for the same mass of the halo (that we should be able to get from dynamics) black hole option would be far more "granular" - i.e. fewer granules and therefore more pronounced density fluctuations - not so?

As per @mvb: if we take the volume of the occupied part of our solar system on one hand; and the theory prediction for the mass/density of BH in a peripheral part of the galaxy disc on the other; we can get an estimate for the number of small BH which could wander within Solar system with a given mass range, e.g. from Pluto and above.Now, if the result is around or greater than 1, it would mean that we should have observed the effects similar to that of a tiny invisible planet in the solar system dynamics. And the fact that we aren't seeing anything like that (fingers crossed) would provide the grounds for the exclusion of such mass/density BH in galaxies. How would it compare with the other limits described here?

A second binary black hole merger detected by LIGO (>5 sigma again): journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.24110314+8 solar masses, merged to a black hole with 21 (+1 solar mass as GW). Similar distance as the previous event.

Two data points still make a poor statistics, but with two events in 3.5 months we should expect several events per year even at the current sensitivity, and a few events per week with the ultimate sensitivity goal. Plus tons of less significant events.

@RBS: With trillions or orders of magnitude more black holes, I guess 1/sqrt(N) fluctuations are not that significant.

After the direct detection of a transient gravitational wave has been replicated thanks to GW151226, one may remark that the mass estimation for the merging compact objects involved relies on general relativity in the strong-field regime and it could be that the classical black hole binaries are not the best model for the source of these GW events, even if they are probably the most simple one! May be GW events will become less boring astrophysical phenomena ;-) once an electromagnetic or neutrino counterpart will have been detected...

The abstract of the PRL paper says that the signal was initially identified within 70 seconds. They also alerted several observatories to search for EM counterparts within a day or so. So clearly, technically they could announce very quickly. I think I've read/heard somewhere (can't remember where) that they're planning to do that once detections become more common and they have more experience with the processing pipeline.

@mfb, but sqrt(N) where N1 = mass of halo / mass of asteroid size BH (BH halo) vs N2 = mass of halo / mass of WIMP (WIMP halo) would be at least 5 orders of magnitude difference. What am I missing? Of course the momentum will have to be take into account too.

Concerning faster news, they could do what SNEWS does for neutrinos: an automatic direct notification with position and time, some first approximate data quickly afterwards, then proper data analysis later. The proper data analysis will go faster once they did it a few times. At some point I also expect them to go to statistical analysis for not-that-significant events ("we observed 100, expected 10 background, ...").

Jester: I am aware that this (BH of right mass trapped in planet/sun/NS) is not a stable in the long term.

I was rather wondering at which mass the turning point would be, assuming GR + Hawking radiation - i.e. between bigger BHs that would eventually accrete all the matter around and eventually consume most of the object they're embedded in and smaller BHs that would be harmless since they would radiate off more energy than they could consume and thus still eventually evaporate. Complicated by the fact that such strong Hawking radiation will probably heavily affect the rate of infalling matter just around that point...

Hope this does not get annoying... Still wondering about what would happen when a PBH gets stuck in macroscopic objects I tried to compute when a PBH could get destructive (i.e. absorbing more matter than it could release in energy)... so I computed when the radiation pressure of hawking radiation would cancel gravitational pull on infalling matter, and it turns out that it only really depends on specific weight of the surrounding matter and its optical depth using this very simplistic approach. Assuming earth core matter and an average optical depth of 10 cm (probably way too low for extremely high-energy gamma rays of such PBHs in question) this would be the case for a PBH of 10^10 kg. For comparison, the mass of a PBH that just evaporated by now would be about 10^11 kg. So I assume pretty much any PBH trapped inside earth would be dangerous, let's hope that there are none or they just eat too slowly or there are other effects reducing their appetite.

I'm sure this is an obvious question but I'm curious. I get that the very early universe was a fog of hydrogen/helium, which started clumping together due to tiny primordial inhomogeneities, thus forming stars. Are we certain that the only outcome for such a clumping is a star, that it could not continue its runaway collapse and go all the way to a black hole without an intervening luminous step? Some process where fusion is not given time to take hold and counter the force of the collapse?

If this is possible, there sure was a lot of matter just floating around available to turn into black holes ...

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Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The posts are often spiced with sarcasm, irony, and a sick sense of humor. The goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...